hp cc +O3 and AC_CHECK_FUNCS

[Kevin Ryde]

With a recent HP cc +O3 on ia64 (at testdrive.compaq.com), all
AC_CHECK_FUNCS claim their function exists, irrespective of whether it
does or not.  I'd like to propose this change to fix that.

The name "conftest_$1" for the second function tries to avoid
conflicting with $1 or any standard function.  Perhaps some fixed
unlikely name would be enough.

        * lib/autoconf/c.m4 (AC_LANG_FUNC_LINK_TRY(C)): Put f=$1 in another
        function, not main, to avoid HP cc +O3 optimizing it away.

c.m4.hp+O3.diff
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